1. Field of the Invention
The present invention relates to inverse emulsions useful as thickener in cosmetic formulations and to the procedure for their preparation.
2. Background of the Art
The inverse emulsions of the invention comprise a polymer obtained by polymerization of an acrylic anionic monomers containing a strong acid functionality (and more specifically a sulfonic functional group), and one or more cationic monomer.
The inverse emulsions of the invention possess high skin and hair compatibility, which makes them particularly suited for the preparation of cosmetic formulations, and exhibit good thickening properties and stability over time.
With the expression “cosmetic formulations” we mean the products normally used for personal care, such as body and face creams, hair gels and lotions, hair coloring and bleaching creams, sunscreen compositions, make-up products, cleansing, moisturizing and perspiring fluids and other products for similar applications.
It is known that a technical problem often encountered in the cosmetic industry is to obtain high viscous formulations (pastes, gels) stable over time and exhibiting high compatibility with skin and hair.
An essential characteristic of the thickeners employed in cosmetic formulations is that they manifest their thickening capability without negatively altering the other properties of the formulations.
In the specialized literature many methods are reported to regulate the rheological properties of different formulations, often including the use of polymers in the form of inverse emulsion (an inverse emulsion is an emulsion containing both an oil-in-water emulsifier and a water-in-oil emulsifier, wherein the aqueous phase is dispersed in the organic phase in very small drops), but the synthetic thickeners for cosmetics of the present invention are never described.
We cite as an example:
EP 503853, wherein an inverse emulsion containing a polymer comprising units deriving from acrylamide, 2-acrylamido-2-methylpropanesulfonic acid and a polyfunctional monomer is described; a disadvantage of the inverse emulsions of EP 503853 is the fact that they contain traces of acrylamide, a toxic substance which is unacceptable by the present European legislative trend;
U.S. Pat. Nos. 6,375,959 and 6,197,287 wherein a procedure for the preparation of cross-linked or branched polyelectrolytes based on strongly acidic monomers and other monomers, but not acrylamide, in the form of an inverse emulsion, is described;
U.S. Pat. No. 6,329,483, wherein copolymers of carboxylic acids and quaternary ammonium compounds and the preparation of gels and emulsions containing the same is described;
US 2001/0023284, wherein copolymers of a neutral monomer (N-alkylacrylamide) with one or more monomers selected among cationic monomers, monomers bearing strongly acidic functional groups and monomers bearing weakly acidic functional groups are described.
The Applicant described in its international application PCT/IT03/00389 thickening inverse emulsions containing an acrylic polymer obtained from the inverse emulsion polymerization of an anionic acrylic monomer containing a weakly acidic functional group, an anionic acrylic monomer having a strongly acidic functional group and a cationic acrylic monomer.
It is still desirable in the cosmetic field to have thickeners in the form of stable emulsion that are able to give stable cosmetic formulations, and that, in addition to a good thickening efficiency in different conditions and ease of use, exhibit an improved compatibility with skin and hairs.